Glass is typically made of silicates that are melted to form a clear, transparent, solid material. The fundamental molecular structural unit of conventional glass is a SiO4 tetrahedron. Ordinary float glass (named for its production process whereby a molten ribbon of glass is floated on molten metal to provide a smooth surface) includes additional amounts of soda (Na2O), usually in the form of sodium carbonate or nitrate during the production process, lime (CaO) and other oxides (usually aluminum and magnesium oxides) to form a soda-lime-silica structure known colloquially as soda-lime glass. Other specialized glass can be prepared by the introduction of other additives and constituents.
Reflections in optical systems occur due to index of refraction (n) discontinuities. Complex layer structures are often deposited on glass substrates in an attempt to compensate for such index discontinuities.
Glass typically has an index of refraction of about 1.46 (i.e., n=1.46), while the index of refraction of air is about 1.0 Thus, given a glass substrate having a refractive index (n) of 1.46 and adjacent air having a refractive index (n) of 1.0, the most desirable index (n) for an AR coating can be calculated as follows:n=square root of (1.46×1.0)=1.23
For an ideal single layer system, the coating would thus have a refractive index (n) of 1.23 and a thickness of around 100 nm for visible applications to 145 nm for photovoltaic applications.
Unfortunately, durable transparent coating materials having a refractive index (n) of 1.23 are not typically available. Because durable AR coatings having an index of about 1.23 are not typically available, those in the art have tried to provide for AR characteristics in other manners as exemplified by U.S. Pat. Nos. 5,948,131, 4,440,882, and 6,692,832 (the entire content of each being incorporated expressly hereinto by reference). For example, it has been proposed to use MgF2 which has an index of refraction of 1.38 at 550 nm as an AR coating on transparent substrates. MgF2 AR coatings tend to not be scratch or etch resistant and are not easily applied. In addition, such MgF2 coatings typically have a reflectance of less than about 2% across most of the visible light wavelengths as compared to about 4% for uncoated soda lime glass.
While the prior techniques used in the art may in fact achieve improvements in the AR properties of transparent substrates, some techniques may be too expensive and/or burdensome. As such, a need still exists in the art for coated articles with improved AR characteristics, and methods of making the same. It is towards fulfilling such needs that the present invention is directed.
Broadly, the present invention is embodied in AR-coated transparent substrates and methods of making the same wherein the AR-coating comprises a fluorinated polymer. In especially preferred forms, the present invention is embodied in anti-reflective transparent articles and methods of making the same having a transparent substrate (e.g., glass), and an anti-reflective coating comprised of a fluorinated polymer on one (or both) surface(s) of the substrate. If coated on both surfaces of the transparent substrate, the fluorinated polymer forming one of the coatings may be the same or different as compared to the fluorinated polymer forming the other coating.
In especially preferred embodiments, the fluorinated polymer is the reaction product of at least one fluorinated acrylic ester monomer which is formed over an anchor layer of a hydrolyzed unsaturated alkyl silane adsorbed onto the substrate surface.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.